Arc chamber



F. E. WOCHNA Feb. 13, 1968 ARC CHAMBER 2 Sheets-Sheet l Filed July 19, 1965 .M w a @n V5. T W5 `A m .gl

F. E. vvocHNA A 3,368,397

Feb. 13, 1968 ARC CHAMBER 2 Sheets-Sheet 2 Filed July 19, 1965 time ,4free/Vey I' power arc directly between the electrodes.

' vide an improved arc chamber.

Francis tE. vWocllna, Torrance, Calif.,l 'assignortoNorth American Rockwell lCorporation, aicorporation"ofl mismos-on DISCLOSURE A blow-down type wind tunnel having'an arc chamber,

nozzle, and 'evacuated test region ist. described. An imnited States ate i" a high energy arcor spark within the arc 'chamberli housing.

' metal liner encompassing the region in which the electric arc occurs. This arc chamber liner in a preferred vembodi-fment is in the. form of a right circular cylinder closed( at onev end and open at the other. The metal 'cylinder is split into two separate longitudinal segments along l a plane extending along the axis ofthe cylinder. l'slect'rical insulation is provided between the two halves ofthe cylinder. Electrical insulation is also provided surrounding the arc chamber vliner thereby preventing electrical n vcontact with other electrically conducting components of proved liner Vfor the arcchamber is provided having two semi-cylindrical metal halves forming a lhollow cylinder for confining an electrical are for gas heating.

Metal electrodes are employed for introducinga heavy current of electricity into the chamber for the arc. The

semicylindrical halves lof the arc chamber are electrically insulated fromeachother, and from the electrodes which enterV fromzthe sidesthereofyso thatA the arc vchamber linerfis electrically isolated from lany possible conduction to ground. In addition, the insulation between lthetwo segments substantially reduces induced currents in the liner during the arc and enhances gas heatingetnciency in thewind'tunnelt` Certain'types of windtunnels -which operate over short i time intervalsemployan electricarc between two elec-- trodes in an' arc chamber to 'generate a highv pressure,

high temperature gas from a relatively lower pressure of lower temperature gas in aminimum time interval with littleV heat loss. The'gasthen expands into an evacuated chamber at high velocity. These tunnels are known as the blow-down-type. Itv has -been found in operation of such wind tunnels that the arc chamberis preferably constructed-of metal having a relatively h igh thermal conductivty so that the large amount 'of heatl dissipated lin the arc chamber by the `arc is Aconducted away from the surface to-minimize damage to the arc chamber.

v improved wind tunnel. f. 1 Itis a further'obiect of this invention to minimize elec- -v A v the wind tunnel. A metal closeout plate is provided over the open end 'of thev arc chamberliner and providesan' orilice lfor the converging section of the nozzle.The elecltrical insulation ybetween the two halves of the arc chamber liner and surrounding the' arc chamber liner aller the electrical characteristics of the arc chamber and virtually eliminate lining erosion. The split liner withA electrical insulation prevents the formation lof undesired' Aarcs to the walls and increases the power levels obtainable from the arc. In addition to increased power and decreased 'gas contamination. the improved linerunex" pectedly provides better velocity characteristics-in the have a single peak of velocity rapidly thereafter.

Thus it is abroad object improved arc chamber.

Itis another object of this of this invention 'to providean invention to vprovide an tric arc erosion.

Itis another object of this invention to provide a vstable are ` vOtticiobjects andmany of the attendant advantages of Arc chamber linings of non-metallic materials have in general been unsatisfactory. Difficulty is also encountered with one piece metallic larc chamber linings due to erosion of the arcchamber lining by' the electric arc with consev quent contamination of the gas generated thereby. Su'ch v added linings have comprised 'continuous sleeves of jmetal grounded to the arc chamber housing. There is also found'.

with s uch arc chamber linings that erratic results are sometinesobtained due to unexplained variations in arc conditions.

. It is hypothesized that in the prior art'arc chambers an arc has formed between one `electrode and the interior of the are chamber. Currentis then conducted through the arc chamber wallto the otherelectrode and a second arc forms between the wall and the second electrode. Thus there are two relatively low resistance, low power arcs instead .of a single relativeiy higher` resistance,higher AIt is therefore a broad object of this invention to pro# ThusI in the practice of this invention according toa I. preferred embodiment there is provideda wind tunnel comprising an arc `chamber housing and a convergingdiverging nozzle connected to the housing leading to a vacuum'chambercontaininglow pressure gas or vacuum. A test model or the likeI provided in the vacuum chamber adjacent the diverging portion of the nozzle is customarily employedin the use of a tunnel. Oppositely disposed electro'des extending into. the arc chamber housing are em#` ployed 'for com'luctingl a Vvery high current for a -very short period of time'froin a bank of capacitorstocreate this invention will'be readily appreciated as the same becomes better understood byreference to the following ydetailed'description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates van overall view of a wind ltunnel incorporating the principles of this invention;

- FIG. 2 is 'a longitudinal sectional view of an arc chamber liner incorporating the principles of this invention;

FIG. 3 is a transverse cross-sectional view ofthe liner jblow-down type substantial erosion ofthe interior of the arc chamber occurred due to arc impingement on the walls of the arc chamber. This not only limited the useful life ofthe arc chamber but also contributed to contaminaentenderse. is, tsss Within the are chamber housing therevis provided a wind.t unnel .in-that a substantial time of constant gas 5 -velocity is obtained, -in contrast toprioi ar t tunnels that with velocity falling oit tion ofthe gas flowing through the wind tunnel with material eroded fromthe arc chamber, The arc chamber liner describedherein substantially reduces metallic contamination of the gas owing through the wind tunnel." 1

It has been hypothesized thatone reason for substantial erosion of the arc chamber in the prior art apparatus has been arcing or sparking between the electrodes and the wall.,jof the arc. chamber with accompanying conduction of electricity parallel to the arev through the wall of the arc chamber. Contact of thearc on the wall not only j y causes erosion ofthe wall but also y.iiverts some .y of the assaaar energy available in the capacitors from the arc and reduces the gas velocities obtainable. By providing an arc chamber liner split along a plane normal to the electrodes into two electrically insulated segments, the tendency for the arc to jump to the chamber liner .has been effectively eliminated since no continuous current carrying path is now available between the two electrodes except through the are therebetween.

Another benefit of the improved electrically insulated liner is in the phase relations f the current and voltage in the power circuit. Previously it has been'found that the current and voltage maxima are somewhat displaced intime from each other so that maximum power eciency is not obtained. With the improved electrically insulated arc chamber liner the current and voltage are in better phase relation thereby lincreasing power efficiency. Random variations in phase relation previously occurred thereby making diicult the prediction of arc energy and hence gas velocity. With the improved liner gas velocity is more accurately controlled by the operator since the phase relations in the arc are substantially constant.

In addition to .higher and m'ore predictable energy levels available in thewind tunnel with less erosion of the arc chamber liner, it is unexpectedly found that the velocity characteristics of the gas generated are appreciably improved. FIGURE 5a illustrates schematically the velocity of gas obtained as a function of time Vin a' prior artv v .wind tunnel that is similar to the tunnel described herein except that the prior z ir't arc chamber is a continuous metallic structure rather than specially insulated as described herein. During operation of such a prior art tunl nel the velocity of gas tlow smoothly .increased to 'a maxilmum and immediately thereafter began to decrease. AThe total useful operating time ofthe tunnel flor a given operation is about 50 milliseconds. In a tunnel incorporating the principles of this invention a velocity Aversus time curve similar.l to that illustrated in FIG-URE 5b is oli` tained. The useful operating time of the tunnel -still rie-'- mains at approximately 50 milliseconds, however the gas -40 as the maximum obtainable in the prior art tunnel and revelocity increases to a maximum approximately the same mains substantially constant near that maximum for api proximately -20 to 30 milliseconds before decreasing. This FIG. l illustrates a wind tunnel of the blow-dowrr typel incorporating the principles of this invention.v As illustrated in .this embodiment there is provided a vacuum chamber lltl'supported by suitable stands lill. The vacuum chamber is connected to a test -section l2 of the wind tunnel also supported by a suitable stand li. During use of the tunnel there is conventionally employed a test specimen 9 seen in. FIG. l through a. window 11S in the test section l2 of the wind tunnel. Connected to the test section is al converging-diverging nozzle ll. Ihe nozzle is in turn connected to an arcchamber housing lid. The

vacuum chamber 10,. test section l2, and nozzle i3 are' in fluid communication with each other. During operation of the tunnel high pressure, high temperature gas generated lin the arc chamber housing M as hereinafter dearc chamber housing to be rolled away from the nozzie section iii for access to the interior of the arc chamber housing. ln a preferred embodiment the wheels R7 are constructed of electrically insulating material so that a complete electric circuit does not exist between the arc chamber housing ltd, the nozzle unit 13 and the stand iid.

The interruption ofA this electric circuit by the insulating l wheelsl'i in addition to the insulated liner apparently helps increase reproducibility of the arcV conditions and hence the operating. conditions of the. wind tunnel. In t an arc chamber employing heavy currents as described herein, large induced currents are encounteredr-Previomtly the resistance of the contact between the arc chamber v housing and the stand varied in a random manner and the induced current through then'ozzlel and ystand le' consequently also varied. Thus` the circuit not only drew some power from the arc, bu't more importantly presented y a randomly varia-ble inductance that modied 'current and 'Y voltage phase relations in an uncontrolled` manner. In-

sulation in this circuit has helped increase arc powerand also p ower level predictability.

Leading into the arc chamber housing are two heavy current .electrodes i8 and t i9. For the purposesV of 'exposition these electrodes are referred to as the upper electrode 18 and the lower electrode H9. Other elements of Y the wind tunnelmay be referred to as upper and lower, l vertical or horizontal, and the like. However, it should be Y. understood the relative orientation 'of components of the wind tunnelA relative to some external reference is not essential tothe practice of this-invention. flu addition for purposes of 'exposition the portion of .the are chamber connected to the nozzle maybe referred to as the front end and the opposite portion of the are chamber is re' 'l upper' and lower shell-like segments 22, each in the vform of a longitudinal half of a hollow right circular cylinder v closed at the rear end and open at the frontend. In a typical embodiment the shell-like segments 22 are copper with a wall thickness of about bis inch. The two shelllike segments are assembled in facing relation along a horizontal plane so as to form a hollow right circular cylinder openat the front end and closed at the rear end. In the horizontal plane axially bisecting the two shell-like segments 22 there is provided a layer of insulating material 23 extending so as to completely insulate one I of the shell-like segments 22 from the other of the shelllike segments 22. It has been found that a layer of class C mica heater plate 0.025 inch thick is satisafctory inu sulation. The insulating material 23 is preferably adhesively bonded to the copper of at least one ofthe shelllike segments to provide ease in manipulation and fabri cation.

Surrounding thetwo shell-lilrefsegments 22 there is provided a layer of insulating material Manch as, for example, glass liber imbedded in a resin such as an epoxy resin. When cured such a material provides a rigid backscribed expands through the nozzle i3, passes through a' the test. section i2, into the vacuum chamberv liti.

For ease in operating the wind tunnel the nozzle 13 and arc chamber housing ld are mounted on a stand le that rolls in grooved tracks (not shown) in the oor so that the nozzle can be separated from the test section for ing for the shelllike segments and maintains them in proper relation despite disassembly and assembly of the arc chamber. lt vis desirable to provide a slight taper, such as, vfor example, one degree, on the outside of the insulation 2d so that a tight lit is readily obt inthe arc chamber housing M. Metal end holsters 25 are also onveniently employed for fitting the liner into the housing. j Circular apertures 2o are provided in the metal seg,- ments 22 at the top and bottom thereof to permit the upf per and lower electrodes ylancl il@ yrespectively (FIG. l) to enter'the arc chamber liner 2i. Theatres of the apertures' 26 are aligned and normal to the plane formed by the insulation 23. A third somewhat smaller aperture 27 is provided through the junction of the two shell-like seg-- ments 22 in the plane of the insulation 23 to permit another electrode to enter the arc chamber as described hereinafter. Electrical insulation 28 is provided around each of the electrode apertures so that the shell-like -segments are insulated from the electrodes. There are also provided apertures 29 in each of the shell-like segmens 22 lwhich are conveniently used for addition of gas to the arc charnber and for a pressure measuring sensor (not shown). It

will be apparent to one skilledin the art that if it is desired to bring electrodes into the Yarc chamber liner through the ends thereof, apertures would b'e provided on the ends and the liner would be split laterally by a layer of insulation to perform in a manner similar to the chamber illustrated in the preferred embodiment.

At the front or open end of the arc chamber liner 21 there'is provided a copper .closeout plate 31. This plate is arranged to seat against the electrical insulation 24 so that it is electrically isolated from the shell-like segments 22. A lip 32 is provided overlapping ther end of the electrical insulation 24 and a portion of the segments 22,to minimize hot gas erosionv of the insulation. A converging nozzle piece 33 is provided in the front closeout plate. rlhisnozzle piece is aligned with the diverging nc-zzle section 13 (FIG. 1) of the wind tunnel. A thin diaphragm 34 of plastic material such as polyethylene terephthalate, polyethylene or the like is provided in the throat 35 of the nozzlel adjacent the nozzle piece 33. This plastic film resists substantial pressures but is designed to rupture and vaporize under.. the high pressures and temperatures encountered during operation of the arc chamber. The throat 35 of the nozzle is, for example, less than 0.1 inch diameter, therefore'a* relatively thin plastic film 34 can withstand high pressures.

,FIG. 4 illustrates a partial sectionof the arc chamber liner 4with electrodes in place. As can be seen in this figure'the upper and lower electrodes 18 and 19 respectively pass through the arcichamber liner 2l and face each other :within the arc chamber. A third electrode 36 enters thearc chamber liner through the aforementioned aperture 27v located in the plane of the insulation 23 separating the .two shell-like segments 22 of the arc chamber liner. Theupper and lower electrodes 18 and 19 are electrically connected to thetwosides respectively of a large bank of capacitors 37 which is capable of providing a large current (in the order of 500,000 amperes peak) for a short period of time for the purpose of sustaining an electric arc within the arc chamber. The term electric arc is used herein.y since the discharge is initiated by other than dielectric` breakdown of gas between the electrodes. The term electric spark might also be used since the discharge lasts for a short time interval, usually less than 300 microseconds.

V'Ahsmall wire 38 such as, for' example, 0.005 to 0.010 inch'tungsten wire, is electrically connected between the upper electrode 18 and the side electrode 36 and is preferably bent to approach butv not touch the lower electrode 19. The upper electrode 18 is electrically connected to a switch?. 39 that is electrically connected to one side of a capacitor 4l. The other side of the capacitor 41 is connectedto the side electrode 36.

'Inorder to operate the wind tunnel incorporating the arc -chamber as described, a plasic diaphragm 34 is placed intheI throat of the nozzle and the arc chamber 14 assembled to the nozzle section 13. The vacuum chamber l is evacuated and ya relatively low pressure of gas such as air, nitrogen or argon is added to the arc chamber by way of one of 'the pressure ports 29. A pressure of less than 1000 p'.s .ila.` inthe arc chamber is customarily used. When good vacuum conditionsv have been obtained in the vacuum chamber the capacitors 37 and 41 are charged by conventional power supplies (not shown).

ATooperate the tunnel the switch 39 is closed, either manually or by automatic means, thereby permitting the capacitor 4l to discharge through the tine wire 38. The current suddenly flowing through the wire 38 is sufficient to explode the wire thereby creating an electrically conduetive vapor within the arc chamber. This electrically conductive material is sutlcient to complete the circuit including the upper electrode 18', the capacitors 37 and the lower electrode 19. The large amount of energy stored in the capacitors 37 is discharged into an electric arc between the upper and lower electrodes 18 and 19 respectively. This electric arc serves to ionize andl elevate the temperature of gas Iwithin the arc chamber to several thousand degrees and consequently increases the pressure of the gas within the 'arc chamber to tens of thousands of pounds per square inch. The increased pressure ruptures the plastic diaphragm 34 thereby permittingvthe gasto expand through the nozzle 13 and ow through the test section 12 at high velocity to the vacuum chamber l0. The high temperature and pressure of the gas within the arc chamber results in a substantial volume of gas flowing through the nozze and the test section of the wind tunnel. A substantial ow is obtainedfor a period of approximately 50 milliseconds with peak gas velocities of several thousand feet per second.

The provision of a split liner in the arc chamber with electrical insulation between the two halves has lincreased the life of the arc chamber liner in a 'blow-down type wind tunnel and substantially reduced contamination of the gas with metal vapor. Thisresut is considered to arise from the insulation between the liner halves which interrupts the prior art circuit through the arc chamber walls, electrically parallel to the arc, which may serve to conduct current and divert the arc from4 between the electrodes so as to ow through the arc chamber wall. Such diversion of the arc, which has apparently been eliminated in the improved liner, erodes the liner at the point of contact of the arc on the liner and contaminates the gas. Severe pitting was encountered in prior art arc chambers, limiting the useful life thereof. The improved liner operates without pitting.

In addition to the electricalinsulation separating the mutually insulated segments of the arc chamber liner there is provided a layer of electrical insulation completely surrounding the metallic arc chamber liner. This electrical insulation isolates the shell-like segments from the remainder of the arc chamber housing and effectively makes both shell-like segments electrically floating. It is hypothesized that during operation of the arc chamber the upper shell-like segment rapidly acquires an electric charge similar to that of the upper electrode due to some electronk ow `therebetween and the lower shell-like segment acquiresfan electric charge similar to that of the lower electrode. These similar charges tend` to repel the arc, thereby further inhibiting arcing between the electrodes and the arc chamber liner. Because of these advances it has been possible in a typical wind tunnel to increase the spacing between the upper and lower electrodes aboutone-third thereby substantially increasing the ratio of arc impedance to the intrinsic circuit impedance of the circuit including the capacitors 37. This places a greater proportion of the stored energy of capacitors in the arc than was available before, therebypermitting operation of the wind tunnel at higher energy levels.

Higher energy levels than in prior art arc chambers are consistently obtainable since the. insulated arc chamber provides electric arcs with reproducible phase relations between the voltage and current. In the prior arc chamber the arc energy as a function of time went through a. series of maxima and minima, thereby degrading the power efficiency. It is found with the improved arccham'- ber that consistent time intervals are obtained between arc initiation and the current and .voltage maxima and a single large peak of arc energy asa function of time is obtained with a few minor small peaks. This not. only permits operation at high power levels, but also increases v currents could not be readily compensated in the prior art arc chambers since the magnitude of the induced current was not constant from one tunnel operation to the next. It is found with the improved electrically insulated j linervthat the induced currents, if any, are substantially 'constant and that adjustment of the phase relation of the current and voltage in the power circuit is readily obtained.

FIG. 6 illustrates another embodiment of arc chamber liner incorporating the principles of this invention. As illustrated in this embodiment there are provided a plurality of metal longitudinal segments 42 of a right circular cylinder with each of the segments 42 separated by a layer of insulation 43 from each adjacent segment 42. The cylinder thus formed is surrounded by a layer of insulation 44 which insulates the metallic portion of the liner from other metallic elements of the arc chamber. An arc chamber liner constructed in the manner illustrated in FIG. 6 functions like the arc chamber described hereinabove and further reduces currents induced in the liner to give better power eiciency and lower circuit nductance. It will be apparent that if electrodes are brought through the end of a cylinder, a series of mutually insulated rings can be employed to form an arc chamber liner incorporating the principles of this invention.

Obviously many variations and modifications of the present invention are possible in light of the above teachings. It is therefor to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described What is claimed is:

l. In an arc chamber for carrying a heavy current in an electric arc for a period of time substantially less than one second including an arc chamber housing, a rst electrode extending within said housing and insulated therefrom, a second electrode extending within said housing and insulated therefrom, and means for providing a large ow of electricity to said first and second electrodes for causing an arc therebetween, an improved arc chamber liner comprising:

first and second metal segments in the form of longit-udinal segments of a hollow cylinder cooperating to at least partially define a hollow cylinder including means for introducing the electrodes into the hollow cylinder from the sides thereof; and

electrical insulating material separating said first and second segments from each other and from the arc chamber housing.

2. In an arc chamber for carrying a heavy current in an electric arc for a period of time substantially less than one second including an arc chamber housing, a first electrode extending within said housing and insulated therefrom, a second electrod extending within said housing and insulated therefrom, and means for providing a large tiow of electricity to said first and second electrodes for causing an arc therebetween, an improved arc chamber liner comprisingi first and second metal segments cooperating to at least partially define a hollow cylinder with a plane separating said first and second metal segments, each of said segments having an electrode receiving aperture nor-mal to the plane separating said first and second metal segments; and

a layer of electrically insulating material in the plane f separting said first and second segments for insulating said first segment from said second Segment.

3. An arc chamber liner comprising:

first and second metal shell-like segments each in the form of a longitudinal half segment of a hollow right circular cylinder open at one end and closed at the other end, said segments being assembled in facing relation to define a hollow right circular cylinder, each of said segments having an aperture extending transversely completely through a side thereof for receiving an insulated electrode, said apertures being axially aligned with each other and normal to the plane separating said first and second segments; and

a layer of electrically insulating material between said first segment and said second segment for insulating said first segment from said second segment.

4. An arc chamber liner as defined in claim 3 further comprising:

a layer of electrically insulating material contiguously surrounding said assembled segments for electrical isolation thereof.

S. In a wind tunnel of the blowdown type including a vacuum chamber, a converging-diverging nozzle connected to said vacuum chamber, and an arc chamber assembly connected to said nozzle, an improved arc chamber assembly comprising:

an arc chamber housing;

an erosion resistant liner in said housing comprising a first metal shell-like segment, a second metal shelllike segment cooperating with said first metal shelllike segment to at least partially define a gas containing volume, and means for insulating said second segment from said first segment;

oppositely disposed electrodes entering said housing and said liner; and

means for insulating said liner from said housing and from said electrodes.

6. In a wind tunnel comprising a vacuum chamber; a

converging-diverging nozzle connected to said vacuum chamber; an arc chamber housing connected to said nozzle.

so that gas can ow from said arc chamber housing through said nozzle to said vacuum chamber; first and second electrodes passing into said arc chamber housing; and a capacitor connected to said first and second electrodes for supplying electric power thereto; the improvement comprising:

an arc chamber liner in said arc chamber housing comprising first and second metal segments assembled in facing relation to at least partially define a gas filled volume, each of said segments having an elec-y trode receiving aperture, said apertures being aligned and normal to a plane separating said first and second segments so that the electrodes are in facing relation within said arc chamber liner;

a layer of electrically insulating material between said first segment and said second segment for insulating said first segment from said second segment; and

a layer of electrically insulating material surrounding said assembled first and second segments for electrical isolation thereof.

7. A wind tunnel comprising;

a vacuum chamber;

a converging-diverging nozzle connected to said vacuu-m chamber;

an arc chamber housing connected to said nozzle so that gas can flow from said arc chamber housing through said nozzle to said vacuum chamber;

a support means for supporting said lare chamber housing;

electrical insulating means for at least partially insulating said arc chamber housing from said support means;

a liner in said arc chamber housing defining a gas containing volume, said liner comprising first and second metal segments cooperating to define a right circular 9 cylinder, and electrical insulation separating said first and second segments;

electrical insulation between said liner and said arc chamber housing;

first and second electrodes passing through said arc chamber housing into the gas containing volume; and

a capacitor connected to said first and second electrodes for supplying electric power thereto,

8. A wind tunnel comprising:

a vacuum chamber;

a converging-diverging nozzle connected to said vacuum chamber;

an arc chamber housing connected to said nozzle so that gas can flow from said arc chamber housing through said nozzle to said vacuum chamber;

a support means for supporting said arc chamber housing;

electrical insulating means for at least partially insulating said arc chamber housing from said support means;

an are chamber liner in said arc chamber housing com prising first and second metal shell-like segments each in the form of a longitudinal half segment of a hollow right circular cylinder open at one end and closed at the other end, said segments being assembled in facing relation to define a hollow right circular cylinder, each of said segments having an aperture through a side thereof, said apertures being aligned and normal to the plane separating said first and second segments;

a layer of electrically insulating material between said first segment and said second segment for insulating said first segment from said second segment;

a layer of electrically insulating material surrounding said assembled first and second segments for electrical isolation thereof;

first and second electrodes passing through the aper tures in said arc chamber liner; and

a capacitor connected to said first and second electrodes for supplying electric power thereto.1

9. A wind tunnel comprising:

a vacuum chamber;

a test section connected to said vacuum chamber;

a converging-diverging nozzle connected to said test section;

an arc chamber housing connected to said nozzle so that gas can ow from said arc chamber housing through said nozzle and said test section to said vacuum tank;

a rupturable diaphragm in the throat of said nozzle;

a support stand for said arc chamber housing;

electrical insulation between at least a portion of said arc chamber housing and said support stand;

an arc chamber liner in said arc chamber housing comd prising first and second metal shell-like segments each in the form of a longitudinal halt` segment of a hollow right circular cylinder open at one end and closed at the other end, said, segments being as sembled in facing relation to define a hollow right circular cylinder, each of said segments having an aperture in a side thereof, said apertures being aligned and normal to the plane separating said first and second segments, each of said segments havingy a.4 recess in an edge thereof forming an aperture in. the assembled segments in the lplane separating said first and second segments;

a layer of electrically insulating material between said first segment and said second segment for insulating said first segment from said second segment;

a metal piece in the open end of said arc chamber liner for closing out the right circular cylinder, said metal piece being electrically insulated from said segments;

a layer of electrical insulating material surrounding said assembled first and secondsegments for insulat= ing said segments from said arc chamber housing;

first and second electrodes passing through the apere tures in the sides of said segmepts of said arc charml ber liner so as to be aligned and have the inner ends thereof spaced apart;

a third electrode passing through the aperture formed between the edges of said segments of said are chamber liner;

a fine conductive wire electrically interconnecting said first electrode with said third electrode inside said are chamber liner;

a first energy storage capacitor having two sides elecu trically connected to said first and second electrodes respectively;

a second energy storage capacitor having two sides electrically connected to said first and third elec trodes respectively, the energy capacity of said second capacitor being suliicient to vaporize said wire; and

a switch in one of the connections of said second capaciu tor to said electrodeso References Cited UNITED STATES PATENTS 2,923,811 2/ 1960 Feldmeyer et al s., 21912l 3,024,350 3/ 1962 Skinner et alo -s 219-75 3,182,496 5/1965 Holderer s m, 73--147 3,242,305 3/ 1966 Kane et ale s 219-'475 DAVID SCHONBERG, Primary Examiner, 

